I like the idea of spacing the factory gusset plates away from the frame some. Might not be as strong as original, but would solve the rust issues.
Might I suggest space it only 1/4" away, use large 1/4" thick "washers" at all the original rivet locations, just use flanged frame bolts and nuts this time, or rivets again if you like. I think 1" is excessive, 1/4" should be all you need.

A proper welding job on the frame and the new gusset plates should be more that sufficient, not sure boxing the frame is necessary or even a good idea??

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-